1. Field of the Invention
The present invention is directed to ultrafine and fine particles and methods for making and isolating such particles.
2. Discussion of the Related Art
Ultrafine particles are defined in the art as having diameters of about 100 nm or less. Such particles are therefore smaller than conventional powders, but larger than typical atom clusters. Ultrafine particles are of great interest due to their numerous applications, including use in the formation of ceramic and metal structures, conductive paths and/or conductive layers in electronic devices and the production of catalysts. For example, the use of ultrafine particles in forming ceramic and metal parts results in small grain size, thus providing the parts with optimal physical properties (e.g., strength and ductility). Also, in electronic devices, the small particles allow creation of finer conductor paths. Variations in processes used to produce ultrafine particles may also produce larger, so-called "fine" particles, which are defined as particles having diameters greater than 100 nm but less than 1500 nm. For many of the applications in which ultrafine particles are desired, fine particles may be equally useful.
It has been difficult, however, to obtain powder of ultrafine and fine particles without experiencing agglomeration into larger, less useful particles. Thus, those skilled in the art have attempted to isolate ultrafine and fine particles in a liquid suspension to prevent such agglomeration.
For example, U.S. Pat. No. 4,872,905 discusses a method of obtaining particles by utilizing a sputtering process and a liquid substrate. The metal particles generated from the target electrode encounter vapors of a heated liquid oil, are covered by the oil vapors, and are then captured by the liquid oil. A complex recovery process is required to obtain a usable end product. In particular, the liquid must be mixed with two solvents, such as kerosene and acetone, to thin out the oil and form a colloidal suspension. The acetone (or comparable solvent having a boiling point lower than both the other solvent and the oil) is removed by heating the solution, and the oil-covered particles then settle in the solution. This separation process may have to be performed up to four times. Moreover, prior to using the particles, the oil covering must be removed, for example by washing the particles in a solvent such as dioxane. Once the oil is dissolved, the particles will tend to agglomerate. Thus, while the method of this patent may offer a way of storing ultrafine or fine particles without agglomeration, it does not provide a means for producing isolated particles in a state that facilitates the actual use of the particles.
Another patent dealing with ultrafine and fine particles, U.S. Pat. No. 4,877,647, describes a method for obtaining a colloidal suspension of metal particles. Vaporized metal in a vacuum is captured by a solvent, which may be present as a gas or liquid. Typically, an external cooling set-up is provided, by which the solvent containing the captured metal atoms and atom clusters can be frozen to the interior of the vaporization vessel. The frozen matrix is slowly heated in the vessel to form a colloidal suspension of metal particles in the solvent. A large excess of solvent is required to obtain the suspension, however, at least 30 to 1000 parts by weight of solvent. Preferred metal loadings range from 0.02 to 0.09 molar. Above this level, the metal particles will tend to agglomerate and precipitate. Thus, ultrafine and fine particles produced according to U.S. Pat. No. 4,877,647 are difficult to utilize in many applications, because they cannot be used separately and distinctly from the large amount of solvent required. Moreover, reduction of the amount of solvent results in undesirable agglomeration, thereby destroying the particles' usefulness.
The need therefore exists for methods of producing ultrafine and fine particles that remain isolated from one another, yet are in a state that facilitates handling and maximizes potential applications.